The present invention relates to transmitters used in process control loops. More specifically, the present invention relates to calibrating temperature transmitters used in process control loops.
Process control transmitters are used to measure process parameters in a process control system. Microprocessor based transmitters include a sensor or sensor input, an analog-to-digital converter for converting an output from the sensor into a digital format, a microprocessor for compensating the digitized output, and an output circuit for transmitting the compensated output. Typically, this transmission is over a process control loop, such as a 4-20 mA current loop. One example parameter is temperature, which is sensed by measuring the resistance of an RTD (Resistive Temperature Device, also called a PRT, Platinum Resistance Thermometer) sensor, or the voltage output of a thermocouple sensor.
One technique for sensing the parameter is by comparison of the resistance of an RTD with an internal reference resistance level in the transmitter which is calibrated when the transmitter is manufactured. For example, resistance of the RTD is measured by connecting it in series with a known reference resistance (R.sub.REF) and applying a current common to both resistances. The resistance of the sensor (R.sub.INPUT) is expressed as follows: ##EQU1## where: R.sub.REFNOM =the nominal resistance of the reference resistance in ohms;
R.sub.CAL1 =a calibration offset in ohms determined during manufacture. This value represents the difference between the calibrated reference resistance (R.sub.REF) and the nominal value of the reference resistance; PA0 R.sub.CAL2 =a user calibration offset; PA0 V.sub.RINPUT =voltage drop across the input; and PA0 V.sub.RREF =voltage drop across R.sub.REF.
Equation 1 is used as part of an auto-zeroing and auto-spanning routine in that errors in measurement of V.sub.RINPUT in the numerator tend to be the same as errors in V.sub.RREF in the denominator and therefore cancel.
The A/D converter digitizes the voltages of Equation 1. The microprocessor receives the digitized values and calculates and compensates R.sub.INPUT according to Equation 1 which is converted into a corresponding sensor temperature value with a look-up table or suitable equation by the microprocessor. The output circuit in the transmitter receives the sensor temperature value and provides an output to the loop as a current level or as a digital value. Unfortunately, R.sub.REF sometimes drifts from the calibrated value of R.sub.REFNOM and R.sub.CAL1, leading to inaccuracies in measurement of temperature.
A typical prior art method of calibrating the R.sub.REF is to connect an external predetermined resistance to the sensor input of the transmitter and have the transmitter enter a calibration mode. The transmitter compares the expected value of the predetermined resistance with the measured value of the R.sub.INPUT and uses the difference to calibrate its electronics by adjusting R.sub.CAL2, the user trim. Typically, the predetermined resistance is an active circuit or a resistance decade box. Active circuitry which simulates resistance is usually inaccurate and typically incompatible with an intrinsically safe environment. Decade boxes are unrepeatable and inaccurate relative to precision transmitters and are unwieldy, requiring the transmitter to be disconnected from the process loop in the field and brought into a calibration lab. They also suffer from drift in resistance value from changes in ambient temperature.
Thermocouples are also used by transmitters to measure temperature. U.S. Pat. No. 4,936,690, entitled "Thermocouple Transmitter with Cold Junction Compensation," and assigned to the same assignee as the present application, describes such a measurement. A thermocouple junction generates a voltage across the junction related to its temperature. However, a junction between dissimilar metals at the terminal block of the transmitter introduces another thermocouple voltage. To measure the thermocouple temperature, it is necessary to measure the temperature of this "cold junction" at the terminal block connection and compensate for this voltage. Typically, in microprocessor based transmitters, a temperature sensor is thermally coupled to the terminal block and connected to the microprocessor of the transmitter through the A/D converter. The microprocessor compensates the thermocouple voltage based upon the cold junction temperature measured by the PRT. Errors in thermocouple temperature measurements arise if the transmitter inaccurately measures the temperature at the cold junction.
Prior art attempts at calibrating thermocouple multiple input transmitters focus on calibrating the transmitter with a known voltage source. However, the source has typically been relatively unstable with respect to a precision transmitter. For example, a precision thermocouple transmitter has a typical accuracy of 0.04 percent of the actual value. Further, little attention has been given to calibrating the cold junction temperature measurement.
The art lacks a self-contained calibration device for adequately calibrating temperature measurements. Further, the art lacks a device that is stable enough to be effectively traceable to a NIST (National Institute of Standards Technology) traceable reference within the precision of a high precision transmitter. As with RTD's, voltage calibrations are inaccurate and not stable enough to be useful as NIST traceable devices (e.g., they are not stable enough to hold their calibration for very long).